How does one explain the power of placebo effects in medication and psychotherapy research? Why do people with medical illnesses who strongly anticipate getting better really do? Response expectancies, those unconscious subjective expectations about substances, processes and social stimuli, actually effect autonomic functioning and may be the key to how we ultimately understand the biochemistry of hope. This volume brings together scientists who have studied this effect in human function and dysfunction in the 1990s and practitioners who have applied these findings to enhance the effectiveness of both pharmacological and psychological treatment. They have extended understanding of how response expectancies account for symptom maintenance, motivation and change in such diverse areas as asthma, substance abuse, sexual dysfunction and smoking. Their often surprising findings point to expectancy modification as a key to enhancing effectiveness of treatment and prevention across settings and theoretical orientations.